Giant viruses cast doubt on a common ancestor of all life forms

Further to the giant viruses we have been writing about (here and here) Caroline Crocker explains at AITSE that the question of whether viruses can be considered life (and if not, what are they?) helped motivate her to pick her major in microbiology:

Therefore, even though viruses do cause us a lot of trouble by hijacking our cells and forcing them to make virus instead of performing the functions of life, technically they are not alive. And, having never been alive, they are not dead. Are they the undead?

Giant viruses are bigger than cells and have only 7% similar genomes to them.

Basically, here’s the problem:

The scientists speculate that these giant viruses evolved from cells (becoming simpler), but since the DNA doesn’t actually match any living cells, they suggest that perhaps “at some point, the dynasty on Earth was much bigger…” So, even though evolutionary theory says that all of life evolved from a common ancestor, becoming more and more complex and diverse, the existence of these giant viruses makes these scientists suggest the opposite scenario.

Bet you didn’t hear it explained that way before: These viruses are overturning definitions that define biology.

Though most people think of viruses as being very harmful to humans, the fact is that the Bacteriophage (Bacteria Eater) virus, as in the preceding video, is actually a very beneficial virus for man.

(Bacteriophage) Viruses in the gut protect from infection – 20 May 2013
Excerpt: Barr and his colleagues,, show that animal mucus — whether from humans, fish or corals — is loaded with bacteria-killing viruses called phages. These protect their hosts from infection by destroying incoming bacteria. In return, the phages are exposed to a steady torrent of microbes in which to reproduce. “It’s a unique form of symbiosis, between animals and viruses,” says Rotem Sorek, a microbial geneticist ,,
“It’s groundbreaking,” adds Frederic Bushman, a microbiologist ,, “The idea that phage can be viewed as part of the innate immune system is original and exciting.http://www.nature.com/news/vir.....on-1.13023

The common view of viruses being parasitic, harmful, comes from the few instances when something goes terrible wrong with a virus (a ‘random’ mutation) and the viruses start attacking us instead of helping us.

“the AIDS virus originated relatively recently, as a mutation from SIV, the simian immuno-deficiency virus. According to Wikipedia, this virus was also benign in its original form:.. Unlike HIV-1 and HIV-2 infections in humans, SIV infections in their natural hosts appear in many cases to be non-pathogenic. Extensive studies in sooty mangabeys have established that SIVsmm infection does not cause any disease in these animals, despite high levels of circulating virus.”http://www.uncommondescent.com.....ent-448372

Nevertheless, Barb raises an important question. Setting aside the matter of whether they are harmful or not, viruses are parasites in the sense of dependency on the host.

So if viruses are dependent on the host and could not replicate without host cells, that would seem to directly imply that they could only exist after cells already exist. In terms of an evolutionary theory, they would have to be downstream from the origin of cells.

Connecting back to this story, if I understand correctly, it seems the idea that viruses are derived from earlier cells is pushing scientists to infer past super-cells of some kind — cells unlike current cells, but more like what we find in the giant viruses (since current cells are so dissimilar to the giant viruses).

Yet, its interesting that these giant viruses must be able to hijack current cells and use them to create new copies of the giant viruses — even though these are larger than the cells they use, and even though there is so little in common in DNA content. Hmm.